In fast engine-driven watercraft equipped with a surface drive, the propeller shaft is pivotable in all directions about an articulation point with a driveshaft coming from the engine or the transmission. The engine and transmission are situated in the hull of the watercraft. When the propeller shaft is pivoted about a vertical plane parallel to the longitudinal axis of the watercraft, the immersion depth of the propeller and thus the conversion of drive energy into thrust is changed, and the position of the longitudinal axis of the hull in the water relative to the horizontal is influenced. This pivoting of the propeller shaft in the vertical plane is referred to as trimming, and the amount of the pivoting as the trim angle. The surface drive reaches its best efficiency at higher speeds and with only a partially immersed propeller. The optimal trim angle is therefore dependent on the speed of the watercraft and is manually set in conventional watercraft, with the corresponding inaccuracy. To an approximation, the speed of the watercraft is proportional to the rotational speed of the propeller shaft or the rotational speed of the drive, which is proportional to the propeller speed. In designs with multiple drive units, each driven by an engine of its own, it is not possible, due to deviations in the engine characteristics and manufacturing tolerances, to set an identical rotational speed for the driving mode in which the watercraft reaches its highest speed. For this reason, an independent change in the immersion depth is necessary to equalize the rotational speed defined for the highest velocity, since that is how the torque of the drive, and thus its rotation speed, is varied. Moreover, the driver is subjected to additional stress from manual trimming, which alongside his other activities also hinders an optimal setting of the trim angle.
In the prior art, an automatic trim control for a surface drive is described, which sets the trim angle automatically depending on the respective driving mode. The driving modes are defined in this case by the position that the watercraft assumes in the water at different speeds.